The present invention relates to a process for the fast and efficient filling of voids of both simple and complex shape which can be carried out at room temperature. In particular, the present invention consists of the use of free flowing thermally expanding and curing powders which are poured into the voids and then heated causing the powder to expand, coalesce and cure and thus filling or partially filling the void space as required. The process according to the present invention is particularly suitable for filling the spaces in, around and between honeycomb or pre-formed foam cores as required to produce a filled or partially filled honeycomb or foam core or any other material used in sandwich panel construction. This process is also a simple and efficient method for filling moulds suitable for use in cellular artefact production. The filled or partially filled mould or honeycomb core can then be cured to produce bonded sandwich panels or moulded cellular artefacts. In sandwich panel construction the core material can be bounded by one or more surface skins and the cured bonded panel can be cut to provide a panel having pre-sealed edges.
Sandwich panels, that is panels produced with rigid faces and lower density material bonded between them have been used in many applications and in high performance versions for around 50 years. The technical advantages of such panels are many but in particular they are structures offering high specific stiffness and modulus, that is, the flexural strength and modulus divided by the density of the panel is greater than that obtainable by the component materials individually. Typical examples of such panels are: a plastic rigid foam bonded between aluminium or glass reinforced plastic faces (skins) for the manufacture of truck bodies; paper honeycomb bonded between wooden skins in the construction of doors; and, in particular, aluminium honeycomb bonded between metal or carbon fibre/resin skins as typically used in the aerospace industry.
In general, high performance panels have the central lower density material (core) bonded to the skins with a thermoset (non melting) adhesive and often employ elevated temperature curing to achieve the highest bonding performance. In the construction of such panels it is often important that the edges of the panels are made solid and smooth by some means to ease joining to other surfaces or further panels or for cosmetic purposes. Other advantages of smooth edges are: safer handling; prevention of accidental damage; and to avoid the ingress of water, dirt and other contaminants which might be detrimental to the performance or external characteristics of the panel. This may be achieved by either assembling the panel before bonding with a solid edge material, such as wood for door panels, or subsequent to bonding, with low density pastes as is usually the case with aerospace honeycomb panels.
Frequently, in panel construction, the core material is not available in a large enough size to make a complete panel or different core materials need to be used within the same panel. In such cases it is often necessary, for structural performance reasons, to join the various core pieces together either before, or during, the bonding to form the panel. This may be achieved by the use of high strength adhesive pastes or adhesive films or tapes which preferably expand before setting on heating during the panel bonding (curing) cycle.
In high performance panels it may also be a requirement that the core material is attached to any solid edges which may be built into the panel. This internal attachment is usually achieved by the use of high performance adhesive pastes or expanding (foaming) films.
In the use of such panels it is often desirable to attach additional structures to them, many of which could be load bearing. For example the attachment of a coat hook to a door sandwich panel or a bolt socket in an aircraft floor panel. In the case of the coat hook which is usually lightly loaded a hook with a sufficiently large flange area may be sufficient to provide the necessary load bearing strength if bonded on the skin surface. For higher loadings a wood block may be inserted as a load attachment point, preferably before, but possibly, after the panel has been produced. In the case of high potential loadings in critical structures, solid high strength blocks can be built into the panel structure in the correct places during manufacture, or special high strength inserts may be bonded into the panel after production and routing out appropriate size holes in the panel to fit the insert, or alternatively the honeycomb in the panel can be reinforced by a high performance thermosetting adhesive or casting paste in those areas which need to be reinforced regardless of complexity of shape. The latter approach, where strong enough, is the most elegant as very specific areas may be reinforced in a honeycomb panel down to the size of an individual honeycomb cell. Furthermore, such reinforcement maybe carried out either before or during the final curing of the panel to avoid cutting of the skins.
In the case of sandwich panels with a plastic foam core it is usually sufficient to carry out these connection, joining and reinforcement operations by the use of a thermosetting paste which stays in position during the curing cycle. Typically this could be a xe2x80x9cthixotropicxe2x80x9d two part epoxy resin based paste. If final sealing and finishing of the edges of the sandwich panel are needed then a similar paste or a syntactic paste could be filled into the edges, smoothed and allowed to cure. A syntactic paste is a term widely used in the Aerospace industry to denote a thermosetting resin either one or two part (needing mixing prior to use) which contains pre-formed hollow small spheres made of glass, carbon, silicates or a variety of plastic materials. A common feature of these micro-spheres is their low density, which is also imparted to the paste and is the prime reason for their use. An additional consideration is the relative ease they give to the cured composition for sanding or smoothing.
In the case of sandwich panels using honeycomb core for high performance ground and marine transport and sports goods and particularly those used in aerospace, where strength, light weight and resistance to degradation is extremely important, these syntactic pastes have been used for most of the attachment and reinforcing needs as described hereinbefore for at least the last 30 years. Typical products of this type are REDUX 252 (RTM) a two pack syntactic epoxy paste available from Ciba Specialty Chemicals PLC, and EC 3524 B/A (RTM) available from the 3M Company.
Where foam materials are used as cores, the edges of the foam are irregular due to cutting and the size of the pore or cell structure within the foam. Where honeycomb is used the situation is usually worse, in that cut or uncut honeycomb edges are extremely irregular and therefore difficult to fill accurately and the size of the gaps to be filled can be almost as large as the individual cell, which itself would be typically up to 6 mm, but is often larger still.
Thus, there exists a particularly difficult physical problem in relation to: the accurate and complete filling of gaps in; the adhering of parts to; and the reinforcing of honeycomb panels. Furthermore, since high performance honeycomb panels such as those typically made from aluminium, phenolic resin coated xe2x80x9cNomexxe2x80x9d(RTM) paper and other metal or fibre based products, are usually used to obtain high performance at minimum weight it is also highly desirable to make excellent connections to the panel component parts and to other materials, where necessary for structural performance reasons, but at minimum weight. In general in aerospace, high performance ground and marine transport and sports applications which use honeycomb, syntactic pastes have been used as the primary means to achieve reinforcement and connection. For the avoidance of doubt direct bonding between the honeycomb and the skin is generally effected by a film or liquid adhesive.
In order for these syntactic pastes to achieve a minimum density for the required strength in these applications it has been necessary to include significant quantities of the hollow micro-spheres. The lower the density required the more micro-spheres need to be incorporated. Since the micro-spheres by definition are lower in density than the resins and hardeners used in the syntactic pastes, they have a tendency to float to the top of the liquid resin or hardener, thus giving a non-uniform, heterogeneous mixture on standing or storage. To prevent this it is common practice to include additional materials which have the general effect of increasing the viscosity of the mixture thus making the whole system very viscous. Typical materials added to prevent micro-sphere flotation are well known to those skilled in the art and include finely divided silica particles such as those sold by Degussa under the trade name Aerosil (RTM) or by the Cabot Group as Cabosil (RTM).
Typical densities of such cured mixed syntactic pastes range from 0.6 to 0.8 gm/ml. These are viscous materials which may be mixed by hand stirring with some difficulty. Alternatively, the materials may be mixed by forcing them through a static mixer head with external pressure applied to the individual components whether they be prepacked in dispenser cartridges or other larger containers. Where the density decreases below 0.6 gm/ml the viscosity becomes so high that kneading by hand or dough mixer may be required. In the latter case, care must be exercised not to break a proportion of the hollow micro-spheres leading to an undesirable increase in density. This consideration also applies to higher density syntactic pastes where they may be pumped through pipes or tubes or other constrictions.
In order to overcome the difficult mixing operations associated with such viscous pastes, single pack syntactics have been produced. Because these are reactive mixtures, that is the resin and hardener have been premixed together, it is necessary to transport and store them under cold, usually xe2x80x98deep freezexe2x80x99 (xe2x88x9218xc2x0 C.) conditions. Even under such cold storage conditions the useable life of the pastes may be severely constrained depending upon the reactivity of the mixture. A further disadvantage of this single pack syntactic approach is the need to allow the mixture to warm to room temperature before removal from its container in order to prevent the condensation of moisture onto the mixture, an effect which is considered highly undesirable as it may have an adverse impact on the wetting properties of the paste, its curing reaction, or even generate excess volatiles during the panel cure cycle. These volatile materials may disturb the position of the paste or even, if excessive, cause damage to the honeycomb core or cause blistering between the core and the skins. An additional disadvantage of the need to warm the paste to room temperature is that those pastes having a high reactivity at room temperature have their usable life decreased. For the latter reason some of the single pack pastes are designed to react quickly only at elevated temperatures, normally the cure temperature used in the panel bonding step. However, whether curing at room temperature or at elevated temperature the single pack pastes, although requiring no mixing by the panel constructor, still suffer from being very viscous tacky materials if the density is in the normal range required for most high performance sandwich panel applications.
Thus the traditional method for filling, fixing, joining, and reinforcing honeycomb to itself and surrounding materials has been the application of these one or two pack syntactic pastes accurately and completely into the complex spaces which exist between the cut honeycomb edges themselves and between them and other surrounding materials and/or filling the honeycomb cells for reinforcement. It is of great importance to fully fill these spaces as failure to do so may create structural weaknesses in the panel and allow the ingress of degredants which ultimately may have the same effect as well as increasing weight.
As may be imagined the task of filling these spaces, many of which may be some centimeter deep, in such a springy and delicate material in the unbonded state of a honeycomb panel, with a very viscous and sticky material is difficulty, messy and time consuming and requires great skill and experience by the operator if it is to be effected properly. It is difficult to maintain a completely uncontaminated environment with such an operation and frequently, a large amount of mixed material is wasted when a particular job is complete or reaction of the mixed material makes it too viscous to use for practical purposes. The operation of filling with these syntactic pastes also results in contaminated equipment, protective clothing, containers and so forth all of which need careful and legal disposal operations. Furthermore, it is impossible practically to avoid all personal contact with these materials in this difficult process whether by touching, breathing or just vapour contact. Whilst in general manufacturers of syntactic pastes attempt to minimise such dangers by careful choice of the materials used in them, the risk of operator sensitivity remains, which is a further disadvantage.
There have been attempts to overcome some of the aforementioned process and technical disadvantages by developing alternative methods to achieve some of the reinforcing and/or joining operations necessary in the manufacture of sandwich and particularly honeycomb sandwich panels.
In one method, resin and hardener components are premixed and the composition is pre-formed into flat sheets (patties) of a suitable thickness for a specific honeycomb panel dimension, covered with a release film on each side and deep frozen. To use the composition the panel manufacturer warms the patty to room temperature, to avoid condensation, removes one release film and presses the patty into the honeycomb panel or vice-versa. While this technique does simplify the application of syntactic materials in sandwich panel reinforcement applications it cannot easily be used for the attachment of honeycomb to itself or to edges and the like. This technique is nevertheless an advantage for reinforcing honeycomb where the honeycomb is the same thickness or a simple multiple of the thickness of the patty. All other disadvantages of frozen premixed material remain with this approach.
In another approach, a pre-formed flexible uniform thickness film of thermosetting, hot curing, foaming, adhesive is utilised. This material must be tailored to size by some cutting technique and placed between the core and other sections of the panel core structure that need to be bonded by the adhesive film. During the thermal curing cycle the film softens, melts, expands and cures. If the film has been correctly tailored and placed the expansion will enable some attachment of the honeycomb and other items such as inserts or edge pieces. These expanding film adhesives cannot be used for reinforcing honeycomb for attachments as they are one of finite thickness, usually around 1.5 mm and would be impossible to position in individual cells or substantial gaps or spaces. Whilst having the advantage over syntactic pastes in general for handling in that they are flexible, non tacky and require no mixing they have strong disadvantages in that cutting to shape and placing in position can be extremely expensive and time consuming and that on expansion and cure they rarely fill all the spaces necessary and hence result in panels having less than the optimum structural strength and rarely fill gaps well enough to avoid the ingress of environmental degradants.
Thus, until now the essential tasks of joining honeycomb, attaching honeycomb to edge pieces and inserts, reinforcing for load bearing attachments and panel edge filling and sealing has been carried out by the skilled use of very viscous adhesives, tacky syntactic pastes or patties or by tailoring of foaming adhesive films as described above.
Thus, it would be a significant technical advantage to develop a method for the production of sandwich panels which are strong, light and durable, and wherein the panel filling operation can be carried out under room temperature conditions; the filling operation has increased efficiency; no mixing is required; the filling operation wastes less filling material; contact hazards are reduced; the filling operation allows for flexibility of filling materials and wherein the components of the final sandwich panels are bonded together in a simple effective manner.
We have now developed a process which overcomes the problems of the prior art processes. The present invention provides a process for the fast and efficient localised filling of sandwich panels which can be carried out at room temperature. Further our invention provides a method for the production of strong, durable, light-weight sandwich panels which are effectively bonded. The present invention additionally provides a method for the manufacture of sandwich panels having a variety of filling materials as well as providing a simple method for the filling and sealing of the panel edges. Our invention consists of the use of thermally expanding powders which are poured into the spaces in, between and around honeycomb or pre-formed foam cores to produce a filled honeycomb which can then be either cured as is or following the addition of a further skin by heating. As can be readily envisaged, such a pourable dry powder can be easily and quickly placed exactly in position, however complex the shape of space to be filled. Furthermore, such powders can be designed to have long shelf lives at ambient temperatures; require no mixing and; provided they contain no very fine particles, cause no pollution and give no waste. Thus, in addition to the ease of use, the process according to the present invention minimises material contamination of the workplace, tools, wetting and operator contact.
In a further aspect of the process according to the present invention a thermally expanding powder can be poured into a released mould, heated to form a coherent low density cured solid object corresponding to the shape of the mould and then demoulded. The benefits of using the process according to the present invention for the production of release moulded objects include ease of use, process efficiency, reduction of material contamination of the workplace, tools, wetting and operator contact.